I've followed (I think) a lot of the good advice on this site for using the existing house wiring to use Vonage throughout the house. Here's my problem:

I'm using the LinkSys Wireless-G Broadband Router with 2 Phone Ports (WRT54GP2). It is hooked into the wall on the second floor. I disconnected the wiring from my gray phone box outside before hooking up the Vonage feed. The telephones on the second floor of my home all work fine with Vonage, but the first floor phones do not (no dial tone or anything).

How did you "disconnect" the network interface box? Did you just unplug the RJ-11 jacks or did you go the draconian route of removing the wires from the screw terminals?

If the latter, I suspect that is your problem.

Chances are, the house wiring is on two circuits and both tie together at the network interface box. If there were two sets of wires (two reds, two greens, two blacks, two yellows) on each screw terminal, that is probably the case.

Make sure these wires are all disconnected from the network box, and then connect them together (red to red, green to green, yellow to yellow, etc.) and that should activate the ground floor jacks.